from the manual
http://d.buffalo.jp/sys/file/1601/72883232891a1181bff82758b1ce/blinking red
A rebuild is in progress. Normally this happens after a
hard drive is replaced in RAID 1 (for HD-QHU3R5, RAID
5/10/1). Do not turn off the power until rebuilding
is complete.
When rebuilding is complete, the
DriveStation will beep three times and the access
LEDs will glow a steady green (not blinking).
During
the rebuild, turning off the power can damage the unit
or corrupt data.
Connection to a computer is still possible during
rebuilding, but the data transfer speed will be slower.
The rebuilding process takes about 4 hours per
TB with
no computer attached. If the drive is in use, the rebuild
will take longer